It is important to have a balanced diet during pregnancy as not eating well can affect the baby's growth. While every woman's body is different, it is generally expected for women to gain weight during pregnancy. However, there can be various reasons why some women may not gain weight at 21 weeks. These include factors such as morning sickness, a high metabolism, or underlying health conditions. It is crucial to consult with your healthcare provider if you are concerned about your weight gain or lack thereof. They can assess your individual situation and provide guidance on how to ensure both you and your baby are getting the necessary nutrients for a healthy pregnancy. They may recommend dietary changes or supplements to support proper fetal development. Remember that maintaining a healthy lifestyle throughout pregnancy is essential for the well-being of both you and your baby.